It is known to transmit power over data lines to power remote equipment. Power Over Ethernet (PoE) is an example of one such system. In PoE, limited power is transmitted to Ethernet-connected equipment (e.g., VoIP telephones, WLAN transmitters, security cameras, etc.) from an Ethernet switch. Typically, DC power from the switch is transmitted over two sets of twisted pair wires in the standard CAT-5 cabling. The same two sets of twisted pair wires may also transmit differential data signals, since the DC common mode voltage does not affect the data. In this way, the need for providing any external power source for the “Powered Devices” (PDs) can be eliminated. The standards for PoE are set out in IEEE 802.3, incorporated herein by reference.
The Ethernet standards specify that there be a first and second pair of twisted wires designated as data pairs and a third and fourth pair of twisted wires designated as spare pairs. The spare pairs are typically not used for PDs that do not require more than the IEEE power limit of 25.5 W. “Power Sourcing Equipment” (PSE) may be any Ethernet device that supplies power to a PD over the data lines. The PSE and PD are typically connected via a standard CAT-5 cable terminated with the standard Ethernet 8-pin (four twisted pairs) connector.
However, PDs are known to exist which require more than the 25.5 W IEEE limit. To minimize PoE power losses along the CAT-5 cable, it is known to connect the data and spare pairs in parallel at both the PSE end and the PD end to reduce resistive losses in the cable. This is shown in FIG. 1.
FIG. 1 represents an Ethernet system using PoE, where the data pairs 12 and 13 in the CAT-5 cable are hard-wired to the spare pairs 14 and 15 in the PSE 18 to conduct the PoE current in parallel along the two paths. This presently violates the IEEE standards for PoE but is done by various users. Since the term “PoE” is sometimes used to identify a system in accordance with the IEEE standards, the system of FIG. 1 may be referred to as “>25.5 W PoE.” The pairs are applied to respective diode bridges 20 and 21 in the PD 22 to combine the currents at the ground and −VIN terminals of a PD controller 24. Although the PoE supply voltage is shown as −55V, the supply voltage can be other values, depending on the system. Since each set of pairs is guaranteed to carry power up to 25.5 W, the combination of the pairs can safely carry higher power if the PD 22 requires it and reduces the IR drop.
The PSE 18 is typically powered by the mains voltage (120 VAC) and uses either an external or internal voltage converter to generate a DC voltage between 44-57 volts. The PoE standards require the PoE to supply a minimum of 37 volts at the PD. The voltage drop along the cable increases with distance.
All pairs in use are terminated at the PD 22 by transformers, such as transformers 23 and 25. It is assumed that the twisted pairs 13 and 15 provide 44 volts and the twisted pairs 12 and 14 are connected to ground. A connection is made to the center tap of the transformers to provide the approximately 44 volts to the PD controller 24. Since the DC voltage is common mode, it does not affect the differential data.
The 44 volts is applied to a DC-DC converter 26 for converting the voltage to any voltage or voltages required by the PD load. The load (e.g., a security camera) is powered by the converter 26 and may communicate with the PSE 18 and any other equipment via the twisted wire pairs.
The IEEE standards require certain low current handshaking procedures between the PSE 18 and PD 22 in order to detect the presence of a PoE-powered device and in order to convey the pertinent characteristics of the PSE 18 and PD 22 prior to the PSE 18 making the full power available to the PD 22.
Below is a simplified summary of the standard handshaking protocol between the PSE 18 and the PD 22.
When a PoE-enabled Ethernet cable is plugged into the PD 22, the PSE controller 27 interrogates the PD 22 to determine if it is PoE-compatible. This period is termed the detection phase. During the detection phase, the PSE controller 27 applies a first voltage-limited current for a fixed interval to the PD 22, via the twisted wire pairs 12 and 13, and then applies a second voltage-limited current for a fixed interval, while looking for a characteristic impedance of the PD 22 (about 25K ohms) by detecting the resulting voltage. If the correct impedance is not detected, the PSE 18 assumes that the load is not PoE-compatible and does not use the PoE capability. The system then operates as a standard Ethernet connection.
If the signature impedance is detected, the PSE controller 27 moves on to an optional classification phase. The PSE controller 27 ramps up the voltage to the PD 22. The PSE controller 27 generates either one pulse (indicating it is a Type 1 PSE) or two pulses (indicating it is a Type 2 PSE). The PD 22 responds to the classification pulses with certain current levels to identify whether the PD 22 is Type 1 or Type 2. A Type 1 PD requires less than 13 W. A Type 2 PD requires up to a maximum of 25.5 W. Various classes (e.g., five classes), each associated with a maximum average current level and a maximum instantaneous current level, within these types may also be identified. A classification resistor may be used, as shown in FIG. 1 connected to the RCLASS pin of the PD controller 24. The PSE controller 27 then may use this power demand information to determine if it can supply the required power to the PD 22, and the PD 22 uses the information to determine if it can fully operate with the PSE 18. There are maximum time windows for the detection and classification phases (e.g., 500 ms).
For higher power equipment, a high power class is designated, which allows the PSE 18 to provide over 25.5 W to the PD 22. With such a high power class, it is recommended to tie the data pairs and the spare pairs together to share the current.
Other types of detection and classification routines and standards may be implemented in the future.
On completion of the detection and classification phases, the PSE controller 27 ramps the PoE output voltage above 42 V, and the FET 30 is closed to directly connect the full PoE voltage to the PD 22. Once an under-voltage lockout (UVLO) threshold has been detected at the PD 22, an internal FET is turned on to create a “Power Good” signal for enabling the DC-DC converter 26. At this point, the PD 22 begins to operate normally, and it continues to operate normally as long as the input voltage remains above a required level.
A low value sense resistor 32 is connected in series with the current path, and its voltage drop is detected by the PSE controller 27. The PSE controller 27 uses the sensed signal to: 1) detect if the PD 22 has become disconnected; 2) detect the actual current through the sense resistor for reporting purposes; 3) detect an overcurrent situation for reporting purposes; and 4) detect whether the PD 22 has reached a hard-stop current limit, which limits the current through the FET 30. The current flags depend on the classification of the PD 22. If ceasing power to the PD 22 is required, the PSE controller 27 turns off the FET 30.
Data may be transmitted over the data pairs and spare pairs using conventional means, such as connecting the transceivers of other devices to the differential terminals of the data pairs' transformers and spare pairs' transformers.
Since the designer of the PSE 18 may not be in control of what devices are connected to the PSE 18, the PSE 18 may be used incorrectly by the user, resulting in damage to the equipment. FIG. 2 illustrates one possible configuration that may result in damage to the equipment.
In FIG. 2, the user has decided to connect one end 34 of a Y-cable 36, shown in FIG. 3, to the 8-pin CAT-5 socket of the PSE 18. The other end of the Y-cable 36 splits into a data pairs end 38 and a spare pairs end 39. The end 39 is shown connected to a CAT-5 socket of a network interface controller (NIC) 42, which is a device arbitrarily selected for the example. The NIC 42 is not configured to use PoE. However, the PD 22 has indicated to the PSE 18 during the detection and classification phase that the PSE 18 should provide the 44 volts on the data pairs and spare pairs. Therefore, the NIC 42 may become damaged and/or the system will shut down due to a flag being raised.
Although such problems may be avoided by providing a separate PSE and CAT-5 cable for the PD 22 and the NIC 42 (or any other equipment), so that an independent detection and classification routine would be performed on each device, such a configuration adds cost and size to the system.
What is needed is a single PSE controller in a PoE system that can either: 1) supply power identically to both the data pairs and spare pairs; 2) or supply power only to the data pairs; or 3) provide no PoE to either pair, where the PSE controller ensures that no PoE voltage will be supplied to a non-PoE compatible device connected to either the data pairs or the spare pairs.